A smart motorway is defined as a concept that utilises technologies and procedures to monitor and respond to fluctuating traffic conditions on our motorways. Smart motorways which are being currently designed and installed have evolved from several years of feedback, lessons learnt and improvements since their first deployment on the M42 motorway in 2006.

According to several studies the financial impacts of congestion on the strategic road network is estimated to cost £2 billion per year, an amount likely to increase further due to the predicted traffic growth up to 2035. The key aim of smart motorways is to reduce congestion and improve journey times by better managing the traffic using roadside technology infrastructure, associated control centres, systems and operational regimes.

Smart motorways support the economy by providing much needed capacity on the busiest motorways, while maintaining safety for motorists and those who work on the roads.

Other benefits of differing types of smart motorway operational regimes have included reductions in accidents and reduced impacts on the environment associated with emissions from stationary or slow moving vehicles.

Variable mandatory speed limits are applied to all types of smart motorways. A mandatory speed restriction refers to the uses of a red ring speed restriction which is legally enforceable.

Traffic conditions are monitored using vehicle detection equipment installed in or adjacent to the motorway at strategic locations. The vehicle detection equipment is linked to a Motorway Incident Detection and Automatic Signalling (MIDAS) system which analyses the data and recognises two differing traffic conditions – queuing traffic or congestion.

Queue protection and congestion monitoring algorithms within the MIDAS system recognise differing traffic conditions and automatically set appropriate signs and mandatory speed restrictions on signals to actively manage traffic conditions. Safety specific intervention is provided by the Queue Protection algorithm which protects the back of queuing traffic and congestion is managed by introducing reduced speed limits to increase traffic throughput.

Following lessons learnt, feedback, driver surveys and simulations, the Smart Motorways All Lanes Running concept was introduced. All Lanes Running schemes permanently convert the hard shoulder into an additional running lane whilst operating with all of the features, systems and signalling associated with controlled motorway schemes.

Message signs are used to provide information to motorists in relation to the road conditions ahead. Where they are not co-located with individual signals they can also display Variable Mandatory Speed Limits.

In order to understand traffic conditions (such as speed and flow) the MIDAS system gathers traffic data from vehicle detector sites which are positioned at strategic points along the motorway network. There are two main types of detectors deployed within smart motorway schemes:

Inductive Loops – the traditional form of vehicle detection on motorways. They are cut into the road surface and measure the change in inductance as a vehicle travels across them to determine vehicle speed and traffic flow.

Radar Vehicle Detection – used increasingly to gather traffic parameters. They are positioned on poles adjacent to the carriageway, reducing construction requirements, removing the requirement for complex traffic management during loop cutting and also reducing ongoing maintenance requirements.

To ensure motorist compliance and to realise scheme operational benefits, Highways Agency Digital Enforcement Camera System (HADECS) equipment is installed within smart motorways. The latest cameras can monitor all lanes of traffic simultaneously and adjust the thresholds at which they capture offences to align with the varying speed restrictions.

NB in 2016, The Commons Transport Committee raised concerns about the safety of smart motorways. These were rejected by the government, but the committee continued to criticise the proposals.

Louise Ellman, Chair of the Transport Committee, said, "...we take real issue with the Government's assertion that all lane running schemes on motorways are no different to other types of roads without hard shoulders. Motorways are a different class of road and drivers have different expectations when using them." Ref Parliament, 29 September 2016.